Throwing power is one of performance parameters of plating solutions that has held electroplaters' attention for many years. Among the many characteristics of electrodeposits, the most commonly specified by end users are macroscopic thickness uniformity and minimum and/or maximum thickness. The capability of a plating bath to give uniform thickness distribution is referred to as having a good throwing power. Deposition of a metal over an entire area of a complex part and leveling of the rough finishes are particularly important. Connectors with grooves, springs, tubing, stamped metal connectors and other complex stamped parts need uniform coverage of metals to meet specified functional requirements. Plate finish standards and end-users' specifications typically give minimum and maximum thickness criteria. Knowing, a priori, whether or not such requirements can be met, is quite challenging because the throwing power, as a parameter specific to each particular plating solution, is not readily available. Therefore, the plating engineer must develop the finishing processes which reproducibly provide acceptable quality. To accomplish this, one must rely on published information about the throwing power of commercially available plating baths. However, little reliable information is available which quantifies the throwing power. This is particularly true for modern electroplating solutions where high current densities usually require high solution agitation.
Since the 1920s an apparatus known as the Haring-Blum (HB) cell has been a standard tool for measuring throwing power. A schematic representation of a Haring-Blum cell, 80, is shown in FIG. 8. It is a rectangular container, 81, with positions for one anode element, 82, and two equi-potential cathode elements, 83 and 84, placed on either side of the anode. Current to the anode element and the cathode elements is provided from a current source 85 via conductors 86 and 87, respectively. By moving the anode element toward one or the other cathode element, various ratios of distances between the anode and the two cathodes may be preselected. The measurement is performed by weighing the deposit on each of the two cathode elements after simultaneous deposition on both. In the case of an "ideal" weight distribution, i.e., both deposits have equal weight. However, the reproducibility and the measurements performed with the Haring-Blum (HB) cell are rather limited. This is primarily due to the lack of well-defined and reproducible hydrodynamics, inability to provide high-speed solution agitation, and inability to attain high current densities.
Many modern electroplating cells employ fluid velocities of several meters per second, while the HB cell or similar equipment has a practical limit at 20-30 cm/s. Solution agitation is typically conducted by means of air bubbling or by magnetic stirrers, e.g. stirrer 88. Therefore, the information obtained under low to moderate agitation is not directly applicable to high-speed processes. The information obtained with the HB cell is limited to low current densities due to the inherent limitations of low solution agitation. Current densities of the order of up to 40 ASF and low solution agitation are amenable for application of the HB cell.
Solution agitation can affect the current distribution through secondary and tertiary current distribution effects and thus influence the throwing power. The electric field and its resulting primary current density distribution are the first order parameters which govern metal deposit distribution, Typically, the current density decreases as the distance from the anode increases. In order to overcome this primary current density distribution effect, commercial plating apparatus are designed to apply the kinetic and/or the transport of matter "adjustments" to the current density.
The kinetics adjustments can be divided into two kinds: the enhanced polarization at retained high current efficiency and enhanced polarization at reduced current efficiency. This results in the so-called secondary current density distribution, definable as a current density distribution which encompasses an additional "resistance" at the interface (polarization), which is not a function of part geometry and/or interelectrode gap. Thus, the strict dependence of current density on the part geometry and solution resistance can be reduced. An additional benefit is obtained when the polarization brings about a non-plating process (e.g. hydrogen evolution) so that the "excess" current is "consumed" without metal deposition.
The transport of matter is the key parameter which can influence throwing power. Generally, the polarization of metal deposition process (assuming the same metal in various chemical environments) is a result of two effects; one is the coordination shell composition and the other is the presence of additives. The coordination shell plays a role in the steps preceding discharge and immediately following the deposition, creating a thin solution layer which is chemically different than the bulk solution. Additives, byproducts and contaminants may also participate in the mechanism of deposition. The magnitude of these effects on metal distribution are a function of the transport of matter.
The transport of matter adjustment is performed with low concentration of depositing metal. Under these conditions, if solution agitation is relatively uniform across the plated part, one obtains the transport limited current at low current density. This results in the so-called tertiary current density distribution and leads to little or no change in the transport limited current density across the plated part, regardless of the magnitude of the excess current.
T. C. Tan undertook to give an electrochemical interpretation to the throwing power, in which electroplating parameters such as electrode polarization and overall voltage are taken into consideration. See T. C. Tan, "Model for Calculating Metal Distribution And Throwing Power of Plating Baths", Plating and Surface Finishing, July 1987, pages 67-71. See also Thiam Chye Tan, "A Novel Experimental Cell for the Determination of the Throwing Power of an Electroplating System", Journal of Electrochemical Society: Electrochemical Science and Technology, Vol. 134, No. 12, December 1987, pages 3011 et seq. Tan has devised his own cell design for throwing power measurements. This multi-compartment cell contains anodes and cathodes covering a range of distances, while mixing is provided by gas spargers. Although the uniformity of agitation has been improved over the Haring Blum cell, it is still limited by low solution agitation rates and restricted solution volume.